In a systematic genomics approach, we will produce a genetic interaction map for the model eukryotic cell, Saccharomyces cerevisiae. The completion of the genome sequence of S. cerevisiae led to the development of a complete and systematic set of "Yeast KnockOut" mutations (the YKO collection). This study defined about 15% of yeast cells as being essential for growth. However, existing methods have not addressed two of the most important unanswered questions About eukaryotic genomes: 1) What pairs of genes interact in a cell? 2) What is the extent of functional redundancy? We know from haphazard small- scale analyses that there are many cases of duplicated genes in which neither single gene is essential, but the double mutant is lethal. Thus much more than 15% of the genome is likely to play an essential role. Also, the patterns of which gene pairs are essential in combination will provide crucial new information on the essential regulatory and metabolic pathways of eukaryotic cells. The proposed project will define genetic conditions under which many genes of known or unknown function are required for viability. Two general approaches will be used: one that generates double mutants using a high-throughput meiotic strategy with a growth readout, and one that generates them using a DNA-mediated transformation strategy with a growth readout, and one that generates them using a DNA-mediated transformation strategy using a microarray readout.. The synthetic lethal results we obtain will be made available through a database we develop. We will also identify or develop bioinformatic tools, as needed to analyze them these new types of data in the context of other gene properties, such as encoded protein similarity and known or proposed function. Similar tools are already available for analyzing two-hybrid interactions and their networks. As part of this effort, we will also make available physical resources that will support the screening of synthetic phenotypes other than lethality. These will enhance the ability of the research community to discover genetic interactions defined by criteria that provide focus on specific biological pathways. The ultimate goal is complete map (a "wiring diagram" of all the genetic interactions of eukaryotic cells. Because yeast and human genes are often conserved in structure and function, many parts of this wiring diagram will be directly applicable to studies of human gene function and dysfunction.